A preferred, though not restricting, field of application of such actuators is e.g. disk valves or ball cocks in the beverage bottling industry. In such disk valves or ball cocks, in at least one end position or in movements of the closing element into or out of the end position, a very high or the maximum switching torque must be often generated by the actuator, which can be subjected to pressure means, e.g. compressed air, on one side against a spring force, or on both sides.
In the generic actuator known from EP 1 222 403 A, both guide rods are loaded by the piston simultaneously and in the same manner to transmit the reaction torque from the switching torque into the housing, independent of the respective direction of the reaction torque depending on the respective direction of the stroke of the piston. Both equally long guide rods are anchored, e.g. welded, in the same cover. During the reciprocating motion of the piston, the free effective bending lengths of the guide rods change inversely to the guide lengths. The free effective bending length is the significant parameter for the bending loads or bending stresses to which the guide rod is subjected mainly in the region of the anchorage in the cover, but also in the region where it penetrates into the guide. Independent of the value of the reaction torque, the bending loads at each guide rod are highest when the free effective bending length is longest. As, depending on the construction and function of the valve controlled by the actuator, one cannot exclude that the reaction torque at the piston is highest when the free effective bending lengths at both guide rods are longest, the risk of wear in the region of the anchorages and also in the mouth regions of the guides and there at the guide rods is high. To allow for this situation, the guide rods are furthermore made of an extremely tough and expensive material in the known actuator. In addition, the piston skirt is reinforced by a metallic outer supporting tube, whereby the number of parts of the actuator is inappropriately increased. As furthermore the cover in which the two guide rods are anchored is not made of the same expensive material as the guide rods themselves for financial reasons. welding of two different materials is problematic, possibly such that no automated welding procedure can be carried out. Nevertheless, the risk of a rupture in the respective welding point remains acute, and this simultaneously in both guide rods as both guide rods are anchored in the same cover and are simultaneously subjected to the highest bending forces when their free effective bending lengths increase together during the operation of the actuator. The guide rods must also be frequently readjusted after welding so that they properly run in the guides.
In the actuator known from EP 1 613 848 B1 (DE 60 2004 001 988 T2), four guide rods are anchored in the housing. One pair of guide rods is anchored in one cover with one end, the other pair is anchored in the other cover with one end, where the free ends of the guide rods do not overlap in the direction of the stroke of the piston. Plastic slide bushes are arranged in the mouths of the guides. Depending on the direction of the reaction torque which depends on the direction of the stroke of the piston, only one pair transmits the reaction torque in the fore stroke, while the other pair transmits the opposite reaction torque in the back stroke of the piston into the housing. While the two guide rods of the one pair take up the reaction torque together, their free effective bending lengths and inversely the guide lengths change in the same manner over the stroke motion, i.e. the sum of the free effective bending lengths and the sum of the two guide lengths of these guide rods transmitting the reaction torque vary depending on the stroke of the piston. Thus, the bending loads of the guide rods are highest when their free effective bending lengths are also highest. This requires a very stable design of the anchorages of the guide rods. The four guide rods which radially have the same distances from the piston axis, which are situated diametrically opposed to each other in pairs each, where one guide rod of one pair each is placed relatively close adjacent to a guide rod of the other pair in the circumferential direction, furthermore inappropriately restrict the radian measure in the piston skirt usable for the connecting links. The actuator consists of many parts, mainly due to the four guide rods, and requires time and cost consuming manufacture.